This invention relates to transducer arrays and more specifically to an apparatus and method for tuning and driving a transducer array over a broad bandwidth.
When a plurality of transducer elements in a transducer array are energized, their mutual interaction tends to degrade the performance of the transducer array. Degradation of performance becomes worse as the operating bandwidth increases. It is desirable to improve the performance of the transducer array in regard to an increase in its operating bandwidth.
As an example, it has been found that when a piezoelectric transducer is tuned with a simple parallel inductor and driven by a constant current source, the resulting dissymmetrical bandpass filter has a modified response. In this response the output quantity is the velocity of the radiating face of the transducer element and the input quantity is the driving electric current. This response, unlike that of a symmetrical filter, seems to be multiplied by the envelope of a rolling-off high-pass filter. This indicates that higher harmonics will be passed. Conversely, when this transducer element is electrically tuned with a simple series inductor and driven by a constant voltage source, the resulting dissymmetrical bandpass filter has a response which seems to be multiplied by the envelope of a rolling-off low-pass filter. This indicates that higher harmonics will be attenuated. This demonstration can be made by velocity and impedance measurements of an actual transducer element in water, or by a lumped equivalent circuit analog, or by a distributed-parameters simulation of the transducer element by a digital computer.
Although the pass band, i.e., frequency range, for velocity of this bandpass filter can be in the neighborhood of an octave in some designs, the pass band for mechanical input impedance, hereinafter called Z.sub.Th, will be only about half an octave, i.e., Z.sub.Th will have an acceptably high magnitude for good array performance over only about half of the octave pass band. Moreover, the electric input impedance, hereinafter called Z.sub.in, will have a sufficiently low phase angle, which implies supplying volt-amps at a high power factor, over only about half of the octave pass band.
One way to broaden the bandwidth over which Z.sub.Th will be sufficiently high for good array performance is to allow the inductance of the tuning inductor to vary during operation. As an example, a minimum of two values of inductance would be needed to increase the velocity-control bandwidth to about one octave. Thus a two-position switch would have to be activated to select one of the two values for the tuning inductor. Since it is usually desirable to locate the inductor in the same housing with the transducer, which may be under water at the far end of a long cable, the switching would have to be done by remote control, i.e., a relay would have to be used in the housing to be controlled by a switching signal provided at a long distance away. This has been considered to be an undesirable feature as it contributes to unwanted arcing in the circuit, noise in the receiving mode and the like. Consequently, a new way is desirable by which the process of remote control switching can be eliminated.